Study on impact factors of tracer gas method in investigations of gaseous pollutant transport and building ventilation

Building ventilation is essential to discharge indoor pollutants and improve indoor air quality for occupant health.Tracer gas method is an efficient way in the field of building ventilation to measure ventilation rate and to evaluate the ventilation performance.Literature shows notable deviation of measured ventilation rate using different tracer gases.In the present study,CFD simulations are carried out to analyze He-,CO_(2)-and SF_(6)-based tracer gas methods.The effects of tracer gas density and release rate on the concentration distribution and ventilation effectiveness are studied.Various application scenarios of different ventilation rates and airflow distribution forms are compared.The results show that the deviation of ventilation effectiveness evaluated by different tracer gases can be above 2-4 times,and the error is introduced by non-passive dispersion.Whether tracer gas dispersion is passive or not depends on the relative importance of density difference driven mass transfer to forced convection mass transfer,which is due to the combined effects of density difference,release rate,and indoor airflow velocity,and can be judged by a dimensionless number θ.Under the geometry and ventilation settings in the present study,the critical value of θ is 1.0 for the error range of 5%,and 2.0 for the error range of 10%.When θ is below the critical value,the gas transport is passive and dominated by the indoor ventilation airflow.A release of tracer gas with smaller release rate and smaller density difference into a stronger indoor airflow behaves more passive.Heavier tracer gas tends to significantly overestimate the performance of upper supply and lower exhaust ventilation,and lighter tracer gas aggravates the overestimation of the performance for lower supply and upper exhaust ventilation.In mechanical ventilation rooms with air change rate of 3.0-6.0 h^(−1),a continuous release of tracer gas SF_(6),CO_(2) or He with release rate above 8 mg/s or source concentration above 8-75 ppm should not be considered as passive.This work clarified the passive and non-passive transport characteristics and mechanisms of various tracer gases,which is helpful for the engineering applications of tracer gas method in building ventilation studies.

The mass and heat transfer process through the door seal of refrigeration

As one of the main reasons causing leakage heat load in a refrigerator,mass and heat transfer through refrigerator door seal is of great importance to be studied.In this paper,a model is presented for numerical simulation of mass and heat transfer process through refrigerator door seal,and an experiment apparatus is designed and set up as well for comparison.A two-dimensional model and tracer gas method are used in simulation and experiment,respectively.It can be found that the relative deviations of air infiltration rate between the simulated results and experimental results were less than 1%,and the temperature difference errors at two special points of the door seal were less than 2.03℃.In conclusion,the simulated results are in good agreement with the experimental results.This paper initially sets up a model that can accurately simulate the heat and mass transfer through the refrigerator door seal,and the model can be used in refrigerator door seal optimization research in the follow-up study.